Pulse Width Modulation (PWM)-type D/A converters that have PWM modulators, and ΔΣ-type D/A converters that have ΔΣ modulators, for example, are used as D/A converters. PWM converters have a problem in that the amount of power consumed increases as the accuracy is improved, and the accuracy suffers as power consumption is reduced. In contrast, ΔΣ modulators are able to achieve better linearity at low clock frequencies than PWM modulators through oversampling and noise shaping. That is, when compared to PWM-type D/A converters, ΔΣ-type D/A converters have the benefit of making it possible to reduce power consumption and to increase accuracy. A ΔΣ-type D/A converter that converts multi-bit input signals into pulse-stream output signals is disclosed in Japanese Unexamined Patent Application Publication 2008-35038, below.
Typically, the outputs of ΔΣ modulators are density signals that are pulses that represent low and high levels. Unlike a PWM waveform, the density pulse determines the low/high level through a feedback circuit within the ΔΣ modulator, so has no periodicity. In a state wherein the ΔΣ modulator output is constant at either the low level or the high level, the internal feedback circuit is in a saturated state. That is, this state is an aberrant state wherein proper, stable operations cannot be insured. In contrast, in a normal ΔΣ modulator, the output cannot be constant at either the low level or the high level. For example, even if the input signal is at 0, still, if the internal workings of the ΔΣ modulator are operating properly, a high-level pulse is output in a given proportion, in order to maintain the stability of the system, meaning that the output value of the ΔΣ modulator does not go completely to 0. Similarly, even if the value of the input signal is at a maximum, the low level pulse can be output with a given proportion, so that the output value of the ΔΣ modulator is not at the maximum value. The result is that a ΔΣ-type D/A converter wherein the output of a ΔΣ modulator is averaged by a filter circuit to output an analog signal is not able to output, for example, as illustrated in FIG. 6, a voltage between 0 V and 0.1 V, which is in the neighborhood of the lower limit of the output voltage (a lower limit-side non-output range) or output a voltage between 2.4 V and 2.5 V, which is in the neighborhood of the upper limit for the output voltage (an upper limit-side non-output range) when the range of the output voltage is between 0 V and 2.5 V.
Some industrial measurement instruments require outputs beginning at 0 V, such as in, for example, 0 V through 1 V or 0 V through 5 V. When using a ΔΣ-type D/A converter, as described above, in such measurement instruments, it is necessary to provide a separate compensating circuit that includes, for example, a gain adjusting circuit, a voltage source, and the like, in order to achieve an output that starts at 0 V. If the accuracy of the compensating circuit is low, then the accuracy of the ΔΣ-type D/A converter will suffer. On the other hand, raising the accuracy of the compensating circuit to match that of the ΔΣ modulator requires a complex structure, increasing costs.
Given this, the present disclosure is to solve the problems such as set forth above in the conventional technology, and the object thereof is to provide a modulator and a ΔΣ-type D/A converter that is able to satisfy easily the required output range without a loss in accuracy.